Current production and drilling strings are generally connected to the sea bottom through a joint composed of a ball joint or a flex joint allowing an angular movement of the order of 10.degree. in any direction. The exception to this rule is provided by the tension-leg platforms installed on the Hutton field in the North Sea and the Jolliet field in the Gulf of Mexico, where the production strings are directly embedded in the wellheads which are combined on the platform vertical.
Such an embedding is advantageous for several reasons. Firstly, it avoids imposing a significant bending on the tubings which are located inside the string. It further reduces the angular clearances thereof. Finally, it is more compact, less costly, and requires less maintenance than a ball joint.
In the case where a drilling riser is used, such a link considerably reduces the wear of the pipes.
The drawback of embedding is that the moments induced by the lateral offset of the platform, a well as by the effect of the sea current, may be extremely high. In order to reduce the bending stresses, which would otherwise exceed the limit allowable in the string, it becomes necessary to give the string or line a variable stiffness over part of the length thereof, more particularly close to the embedding. This is achieved by means of a variable stiffness element.
The element may be designed so that the curvature caused is substantially constant over the total length thereof. This requires that the stiffness in bending (EI) evolves in a precise way along the element.
The data of the problem are the following:
A.sub.B =the angle at the base of the string in the case of a non stiff joint, PA1 (EI).sub.R =the stiffness in bending of the string, PA1 (EI).sub.O =the stiffness in bending of the upper end of the variable stiffness element, and PA1 M.sub.O =the maximum allowable moment at the joint between the string and the element PA1 If T=the tensile force on the joint and C.sub.O =the shear force on the joint and by putting ##EQU1## R.sub.e =the minimum allowable bending radius of the element (=(EI).sub.O /MO) and PA1 L=the length of the element. PA1 a) a first layer of fiber composite is wound in a helicoid way with an angle equal, in absolute value, to alpha with respect to the axis of an initial pipe; PA1 b) a second layer of fiber composite is wound in a helicoid way with an angle equal, in absolute value, to beta with respect to the axis of the pipe; PA1 c) one or several draped composite underlayers are deposited, the underlayers having longitudinal dimensions which grow shorter as the underlayer is located further from the axis of the pipe; PA1 d) one or several layers of the type of the first or of the second layer are wound, and PA1 e) the assembly is subjected to a cross-linking stage. PA1 a) a first layer of fiber composite is wound in a helicoid way with an angle equal, in absolute value, to alpha with respect to the axis of an initial pipe; PA1 b) a second layer of fiber composite is wound in a helicoid way with an angle equal, in absolute value, to beta with respect to the axis of the pipe; PA1 c) essentially longitudinal fibers are continuously deposited so as to constitute composite underlayers, said underlayers having longitudinal dimensions which grow shorter as said underlayer is located further from the axis of the pipe; PA1 d) one or several layers of the type of the first or of the second layer are wound; and PA1 e) the assembly is subjected to a cross-linking stage. PA1 g) a ring is positioned at each end of a length defining an underlayer forming the variable stiffness element, with the ring bearing spikes distributed on the periphery thereof; PA1 h) the fibers or fiber plies forming an underlayer are spooled by turning round on the spikes; PA1 i) each end of the fibers or fiber plies is bound by circumferential winding, so that the protuberances due to the returns on the spikes are located outside the part of constant thickness of the underlayer located between the two rings; PA1 j) the protuberances due to the return on the spikes are removed; PA1 k) at least one of the two rings is displaced by a pitch equal to the offset that is wanted between the underlayers forming the variable stiffness element; PA1 l) stages g) to k) are repeated until a number of underlayers corresponding to the variable stiffness desired for the variable stiffness element is obtained. PA1 m) a ring is positioned at each end of a length defining an underlayer bearing the variable stiffness element, said ring having spikes distributed on the periphery thereof; PA1 n) the fibers or fiber plies forming an underlayer are spooled by turning round on the spikes; PA1 o) the assembly is subjected to a cross-linking stage; PA1 p) the protuberances due to the returns on the spikes are removed; PA1 q) at least one of the two rings is displaced by a pitch equal to the offset that is wanted between the underlayers constituting the variable stiffness; PA1 1) stages m) to q) are repeated until a number of underlayers corresponding to the variable stiffness desired for the variable stiffness element is obtained.
To understand these designations better, see FIG.7 of the present application.
It can be demonstrated that the following relations are approximately correct: EQU C.sub.O =K.sub.R .multidot.M.sub.O ( 1)
The length (L) necessary for the element ##EQU2##
The angle (A.sub.e) by which the element must bend is ##EQU3##
The required evolution of the stiffness in bending (EI) along the element ##EQU4##
The maximum moment at the lower end of the element ##EQU5##
It can be deduced from Equation (2) that the lower the mimimum stiffness in bending (EI).sub.O of the element is, the shorter the element may be.
Equation (5) shows that the maximum moment transmitted to the foundation is in direct relationship with the length (L) of the element and with the allowed bending radius (R.sub.e). It is therefore desirable to make this element as supple as possible.
In the case where the string is made of a supple material such as, for example, a composite material (carbon fibers/glass fibers/resin) and if the element consists of a stiff material such as steel for example, it is possible that the allowable moment (M.sub.O) at the joint between them is much lower in the string than in the element.
There are two possible solutions.
The element may be made longer than it needs to be for itself. The alternative consists in introducing a transition joint several meters long and of constant section between the string and the variable stiffness element.
The optimum solution in order to avoid having to introduce a transition joint between the string and the variable stiffness element is to provide the element with an upper end at least as supple in bending as the string itself.
It is the same way in the case of a string made of a stiff material such as steel for example, and of an element consisting of a supple material such as, for example, a composite material (carbon fibers, glass fibers, resin).
In the description hereafter of the present invention, what is called composite material is a material comprising fibers, such as glass fibers, carbon fibers, aramid fibers, coated in a matrix such as a thermoplastic or a thermosetting matrix, for example an epoxy resin.
What is called draping is the deposition of a set of resin-coated fibers by applying a contact pressure. The fibers may be pre-impregnated with the resin or coated with resin after being deposited.
Draping may be achieved with unidirectional plies made of fibers in a single direction or with fabrics whose fibers generally have two directions forming an angle of 90.degree..
The standard part of a pipe is defined as generally consisting of several layers made of fibers wound at at least two angles with respect to the axis of an initial pipe.
Various processes are known for manufacturing pipes whose length is practically insensitive to variations in the operating conditions when they are used in the field of hydrocarbon prospecting and development, and more particularly the following three patents.
Patent FR-2,557,254 thus describes a process allowing production of flexible pipes whose length is practically insensitive to the effect of the internal pressure.
Patent FR-2,267,840 describes a process for manufacturing composite pipes whose length practically does not vary under the effect of the internal pressure variations.
Patent FR-2,648,535 allows an optimization of the characteristics of composite pipes such as those described in the above-cited two patents.
But these prior processes do not allow obtaining a pipe or an element whose stiffness varies.
French patent FR-2,616,858 describes an element whose stiffness in bending varies and is close to that of a string made from a composite material. According to this prior art document, the element achieved thereby consists of shells made of metal or possibly of other materials, arranged in one or several layers around the pipe. The dimensions of each shell and the lay-out of the various shell layers are such that the evolution of the stiffness of the pipe and the shell assembly along the element is that which is required by a variable stiffness riser base, while respecting the limit stresses of the different components of the element.
However, the prior elements do not allow a stiffness in bending values substantially equal to those of a string made from a composite material.
French patent application FR-2,641,841 describes a process for the continuous winding of fibers round a spindle allowing to assemble a metallic tip to a pipe, but which does not allow to obtain spooled layers of variable lengths.